This invention relates to indication generator/motor systems and more particularly to an induction generator/motor system whose effective power output and reactive power output are controlled by controlling the secondary current while operating a pump hydraulic turbine at variable speeds.
As an induction generator/motor system utilizing an induction machine with its secondary side connected to a power converter, there have been available a system as disclosed in Japanese Patent Publication No. 53-7628 wherein effective power is controlled by controlling a secondary current component which is in phase, as viewed from the stator, with stator voltage and another system as disclosed in Japanese Patent Publication No. 57-60645 wherein reactive power is controlled by controlling a secondary current component which is 90.degree. out of phase, as viewed from the stator, with the stator voltage. These systems are respectively suitable for use as a power factor regulator and a power regulator which are capable of making a quick response while preventing hunting and pull out.
When applying these systems to a generator/motor system of large capacity such as a variable speed pump-up generator system, the capacity of thyristors constituting a power converter connected to the secondary side becomes large. A cyclo-converter used as the power converter, especially, a non-circulating current type cyclo-converter is suited for the purpose of reducing the required capacity of the thyristors.
In controlling the secondary current of the induction machine, when voltage on an AC power line system varies greatly an account of an accident in the AC power line system, a DC transient current will flow in the primary side of the induction machine. Because of this DC transient current, a rotating frequency current component of a frequency equivalent to a revolution number is induced in the secondary side of the induction machine and superimposed on a slip frequency current component. Where the non-circulating type cyclo-converter is used as the power converter for secondary excitation, the polarity or direction of current conduction in the cyclo-converter is typically switched by first interrupting one conductive direction and after lapse of a thyristor turn-off time, applying a current conduction signal for the other conductive direction. Therefore, in a phase where a superimposed current of the rotating frequency current component and slip frequency current component flows in a direction of the conduction polarity of the cyclo-converter, the transient current due to the rotating frequency current component is permitted to conduct without raising any problem. But when the superimposed current is directed in opposition to the conduction polarity, its conduction is prevented, resulting in the open state of the cyclo-converter and consequent interruption of the secondary current, and this causes a large induced voltage to develop in an opened secondary winding to maintain magnetic balance on the secondary side. Disadvantageously, the induced voltage damages the cyclo-converter and the secondary winding of the induction machine.
It has been proven by analysis and experiments conducted by the present inventors that the induced voltage in the secondary side extremely exceeds a rated voltage of the cyclo-converter determined by a normally set maximum slip. Prevention of the occurrence of induced overvoltage is therefore needed.
Generally, a method of protecting the thyristor power converter against overvoltage is to connect a non-linear resistor element in parallel with the output circuit of the thyristor converter, the resistor element having such a characteristic as to decrease its resistance under the application of voltage in excess of a predetermined voltage. In a conventional system applied with this method, when the flow of the superimposed current is in opposition to the conduction polarity of the power converter, the superimposed current flows through the non-linear resistor element to suppress the occurrence of the overvoltage in the power converter but at the same time, it generates a large heat loss in the non-linear resistor element. Accordingly, the non-linear resistor element is required to have a very large heat capacity and in fact this method is unpractical.
Another method for protection against overvoltage is such that a pair of thyristor short-circuiting switches connected in anti-parallel relationship are connected in parallel with the output circuit of a power converter, and upon generation of an excessive voltage in the power converter, the thyristor short-circuiting switches are turned on to prevent the occurrence of an overvoltage in the secondary circuit of the induction machine. In the induction generator/motor based on the secondary current control, the secondary current of the induction machine is detected and fed back for controlling. However, the short-circuiting of the secondary circuit of the induction machine upon the generation of the overvoltage according to this protection method will make the value of the current flowing through the secondary circuit independent of the output current of the power converter. Further it is not possible to provide suitable means for positively turning off the short-circuiting of the secondary circuit and therefore after the short-circuiting, the secondary current control based on feedback becomes impossible and the induction machine must be stopped for operating temporarily. Disadvantageously, this method can not therefore be employed for an induction generator/motor which is required to operate continuously with high reliability.